As FHEM does not use a default time format, you need to configure a timeschema as shown above.

The cool thing here is, that FHEM stores historic data and by using the from_beginning switch you can tell telegraf to import all data. Depending on the size of your logfiles, that can take quite long.

I recommend to perform a --debug run to import all data, then change from_beginningto false and then restart Telegraf.